Hydrogen detection method using gas sensor having a metal oxide layer

ABSTRACT

A hydrogen detection method using a gas sensor, in which the gas sensor includes a first conductive layer; a second conductive layer including a first region having a first thickness and a second region having a second thickness larger than the first thickness; a metal oxide layer disposed between the first conductive layer and the second conductive layer; and an insulation layer covering the first conductive layer, the second region of the second conductive layer, and the metal oxide layer and not covering the first region of the second conductive layer. The hydrogen detection method includes allowing a gas to come into contact with the first region of the second conductive layer; and detecting a hydrogen gas contained in the gas by detecting a decrease in resistance between the first conductive layer and the second conductive layer.

CROSS-REFERENCE OF RELATED APPLICATIONS

This application is a Divisional application of U.S. patent applicationSer. No. 15/451,579, filed on Mar. 7, 2017, which claims the benefit ofJapanese Application No. 2016-062483, filed on Mar. 25, 2016, the entiredisclosures of which Applications are incorporated by reference herein.

BACKGROUND 1. Technical Field

The present disclosure relates to a gas sensor.

2. Description of the Related Art

Japanese Unexamined Patent Application Publication No. 59-58348 hasdisclosed a gas sensor which detects the presence of a hydrogen gas bythe change in resistance. This gas sensor includes a material formed byadding palladium (Pd) and a glass to tantalum pentoxide (Ta₂O₅) andplatinum (Pt) electrodes sandwiching the material.

In Sensors and Actuators A 172 (2011), p. 9-14, a Pt/Ta₂O₅ Schottkydiode for hydrogen sensing has been disclosed. In this Schottky diode, ahydrogen molecule is dissociated into hydrogen atoms on the surface of aPt catalyst.

SUMMARY

In one general aspect, the techniques disclosed here feature a gassensor which comprises: a first conductive layer; a second conductivelayer including a first region having a first thickness and a secondregion having a second thickness larger than the first thickness; ametal oxide layer disposed between the first conductive layer and thesecond conductive layer, the metal oxide layer including a bulk regionand a local region surrounded by the bulk region, a degree of oxygendeficiency of the local region being higher than that of the bulkregion; and an insulation layer covering the first conductive layer, thesecond region of the second conductive layer, and the metal oxide layerand not covering the first region of the second conductive layer.

It should be noted that general or specific embodiments may beimplemented as a system, a method, an integrated circuit, a computerprogram, a storage medium, or any selective combination thereof.

Additional benefits and advantages of the disclosed embodiments willbecome apparent from the specification and drawings. The benefits and/oradvantages may be individually obtained by the various embodiments andfeatures of the specification and drawings, which need not all beprovided in order to obtain one or more of such benefits and/oradvantages.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a cross-sectional view showing a structural example of a gassensor according to a first embodiment;

FIG. 1B is a plan view showing a structural example of the gas sensoraccording to the first embodiment;

FIG. 2 is a cross-sectional view showing the structure of a gas sensoraccording to a modified example 1 of the first embodiment;

FIG. 3 is a cross-sectional view showing the structure of a gas sensoraccording to a modified example 2 of the first embodiment;

FIG. 4A is a cross-sectional view showing the structure of a gas sensoraccording to a modified example 3 of the first embodiment;

FIG. 4B is a plan view showing the structure of the gas sensor accordingto the modified example 3 of the first embodiment;

FIG. 5A is a cross-sectional view showing a method for manufacturing thegas sensor according to the modified example 3 of the first embodiment;

FIG. 5B is a cross-sectional view showing the method for manufacturingthe gas sensor according to the modified example 3 of the firstembodiment;

FIG. 5C is a cross-sectional view showing the method for manufacturingthe gas sensor according to the modified example 3 of the firstembodiment;

FIG. 5D is a cross-sectional view showing the method for manufacturingthe gas sensor according to the modified example 3 of the firstembodiment;

FIG. 5E is a cross-sectional view showing the method for manufacturingthe gas sensor according to the modified example 3 of the firstembodiment;

FIG. 5F is a cross-sectional view showing the method for manufacturingthe gas sensor according to the modified example 3 of the firstembodiment;

FIG. 5G is a cross-sectional view showing the method for manufacturingthe gas sensor according to the modified example 3 of the firstembodiment;

FIG. 5H is a cross-sectional view showing the method for manufacturingthe gas sensor according to the modified example 3 of the firstembodiment;

FIG. 5I is a cross-sectional view showing the method for manufacturingthe gas sensor according to the modified example 3 of the firstembodiment;

FIG. 6 is a graph showing the state of the gas sensor according to themodified example 3 of the first embodiment;

FIG. 7 is a cross-sectional view showing a gas sensor according to amodified example 4 of the first embodiment;

FIG. 8A is a cross-sectional view showing a gas sensor according to amodified example 5 of the first embodiment;

FIG. 8B is a plan view showing the gas sensor according to the modifiedexample 5 of the first embodiment;

FIG. 9A is a circuit diagram showing an evaluation system of the gassensor according to the modified example 5 of the first embodiment;

FIG. 9B is a graph showing an evaluation result of the gas sensoraccording to the modified example 5 of the first embodiment;

FIG. 10A is a schematic view showing a resistive state and an oxygendefect state of a gas sensor;

FIG. 10B is a graph showing the relationship between the resistance ofthe gas sensor and the number of oxygen defects;

FIG. 11 is a graph showing the relationship between an exposure time ofthe gas sensor to a hydrogen-containing gas and the number of hydrogenmolecules which reach a local region;

FIG. 12 is a graph showing the relationship between a hydrogen gasconcentration and the thickness of a second conductive layer at whichthe resistance of the gas sensor is changed after one second from thestart of gas introduction; and

FIG. 13 is a side view showing a structural example of a fuel-cellvehicle according to a second embodiment.

DETAILED DESCRIPTION

Underlying Knowledge Forming Basis of the Present Disclosure

Through intensive research carried out by the present inventors, thefollowing problems were discovered. In a related gas sensor, in order toimprove the sensitivity to detect a gas containing a hydrogen atom, agas detection element is heated to 100° C. or more. Hence, the powerconsumption of a related gas sensor is at least approximately 100 mW.Accordingly, when a gas sensor is always used in an ON state, a problemin that the power consumption is remarkably increased may arise.

A gas sensor according to one aspect of the present disclosure is ableto rapidly detect a hydrogen-containing gas and is also excellent inelectrical power saving.

Hereinafter, embodiments of the present disclosure will be describedwith reference to the drawings.

Incidentally, in the drawings, an element having substantially the samestructure, operation, and effect as that described already will bedesignated by the same reference numeral, and the description thereofwill be omitted. In addition, the numerical value, the material, thecomposition, the shape, the film forming method, and the like, whichwill be described below, are shown by way of example in order toparticularly explain the embodiment of the present disclosure, and thepresent disclosure is not limited thereto. In addition, the followingconnection relationship between the constituent elements to be describedbelow is shown by way of example in order to particularly explain theembodiment of the present disclosure, and the present disclosure is notlimited thereto. In addition, among the constituent elements of thefollowing embodiments, the constituent element which is not described inthe independent claim representing the most generic concept will bedescribed as an arbitrary constituent element.

First Embodiment

[Structure of Gas Sensor]

A gas sensor according to a first embodiment is a gas sensor having ametal-insulator-metal (MIM) structure in which a resistive film (metaloxide layer) is sandwiched by metal films. This gas sensor usesself-heating and gas sensitivity in a local region formed in theresistive film. Accordingly, the gas sensor can detect ahydrogen-containing gas without performing heating by a heater. In thiscase, the hydrogen-containing gas is a generic name of a gas formed ofmolecules each having at least one hydrogen atom, and for example,hydrogen, methane, and an alcohol may be mentioned.

FIG. 1A is a cross-sectional view showing one structural example of agas sensor 100 according to the first embodiment.

FIG. 1B is a plan view showing one structural example of the gas sensor100 according to the first embodiment. The cross-section of FIG. 1Acorresponds to a cross-section viewed in an arrow direction along thesection line IA-IA of FIG. 1B.

The gas sensor 100 includes a substrate 101, a first insulation layer102, a first conductive layer 103, a resistive film 104, a local region105, a second conductive layer 106, and a second insulation layer 107.In this case, the resistive film 104 is one example of a metal oxidelayer.

The first insulation layer 102 is disposed on the substrate 101, and thefirst conductive layer 103 is disposed on the first insulation layer102. The second insulation layer 107 is disposed on the first insulationlayer 102 and the second conductive layer 106.

The first conductive layer 103 and the second conductive layer 106 aredisposed so that the respective principal surfaces thereof face eachother above the first insulation layer 102. The resistive film 104 isdisposed in contact with the principal surface of the first conductivelayer 103 and the principal surface of the second conductive layer 106.

In the second insulation layer 107, an opening 108 is provided so thatthe second conductive layer 106 is in contact with a gas to beinspected. In other words, the second insulation layer 107 covers thefirst conductive layer 103, the second conductive layer 106, and theresistive film 104. However, at least a part of the upper surface (theother surface facing the principal surface described above) of thesecond conductive layer 106 is exposed without being covered with thesecond insulation layer 107.

The thickness of an exposed first portion 1061 of the second conductivelayer 106 is smaller than the thickness of a second portion 1062 of thesecond conductive layer 106 covered with the second insulation layer107. The second conductive layer 106 is a single layer in which thethickness of the first portion 1061 and the thickness of the secondportion 1062 are different from each other and may be formed, forexample, by removing a part of a flat plate having the thickness of thesecond portion 1062 so as to form the first portion 1061.

The resistive film 104 is provided between the first conductive layer103 and the second conductive layer 106. The resistance of the resistivefilm 104 is changed in accordance with an electrical signal appliedbetween the first conductive layer 103 and the second conductive layer106. In particular, the resistive state of the resistive film 104 isreversibly changed between a high resistive state and a low resistivestate in accordance with a voltage (potential difference) appliedbetween the first conductive layer 103 and the second conductive layer106. In addition, the resistive state of the resistive film 104 ischanged from a high resistive state to a low resistive state in responseto a hydrogen-containing gas to be brought into contact with the secondconductive layer 106.

The local region 105 is formed from the same metal oxide as that of theresistive film 104. The local region 105 is disposed in the resistivefilm 104 so as to be in contact with the second conductive layer 106 andis not in contact with the first conductive layer 103. The degree ofoxygen deficiency of the local region 105 is high as compared to thedegree of oxygen deficiency of the periphery thereof (that is, a bulkregion of the resistive film 104). The degree of oxygen deficiency ofthe local region 105 is reversibly changed in accordance with anelectrical signal to be applied between the first conductive layer 103and the second conductive layer 106. In addition, the local region 105is changed from the state of a low degree of oxygen deficiency to thestate of a high degree of oxygen deficiency in response to ahydrogen-containing gas to be brought into contact with the secondconductive layer 106.

The local region 105 is a minute region in which a filament (conductivepath) formed from oxygen defect sites is assumed to be generated andlost. It is believed that the change in resistance of the resistive film104 occurs when the filament is generated or lost by anoxidation-reduction reaction performed in the local region 105.

In addition, in the present disclosure, the “degree of oxygendeficiency” of a metal oxide indicates the rate of a deficient amount ofoxygen of the metal oxide to the amount of oxygen of an oxide having astoichiometric composition formed from the same elements as those of themetal oxide (in this case, the deficient amount of oxygen is obtained bydeducting the amount of oxygen of the metal oxide from the amount ofoxygen of a metal oxide having a stoichiometric composition). If thereare a plurality of metal oxides having stoichiometric compositionsformed from the same elements as those of the metal oxide, the degree ofoxygen deficiency of the metal oxide is defined by one metal oxidehaving the highest resistance among the metal oxides havingstoichiometric compositions. A metal oxide having a stoichiometriccomposition is stabler than a metal oxide having another composition andhas a higher resistance than that thereof.

For example, when the metal is tantalum (Ta), since an oxide having astoichiometric composition by the above definition is Ta₂O₅, it can berepresented by TaO_(2.5). The degree of oxygen deficiency of TaO_(2.5)is 0%, and the degree of oxygen deficiency of TaO_(1.5) is(2.5−1.5)/2.5=40%. In addition, a metal oxide having an excess amount ofoxygen has a negative degree of oxygen deficiency. In addition, in thepresent disclosure, unless otherwise particularly noted, the degree ofoxygen deficiency can be represented by a positive value, 0, or anegative value.

An oxide having a low degree of oxygen deficiency has a high resistancesince being closer to an oxide having a stoichiometric composition, andan oxide having a high degree of oxygen deficiency has a low resistancesince being closer to a metal forming the oxide.

An “oxygen content” is the rate of the number of oxygen atoms to thetotal number of atoms. For example, the oxygen content of Ta₂O₅ is arate (O/(Ta+O) of the number of oxygen atoms to the total number ofatoms and is 71.4 atomic percent. Hence, the oxygen content of an oxygendeficient-type tantalum oxide is larger than 0 and smaller than 71.4atomic percent.

The local region 105 is formed in the resistive film 104 by applying aninitial break voltage between the first conductive layer 103 and thesecond conductive layer 106. In other words, the initial break voltageis a voltage to be applied between the first conductive layer 103 andthe second conductive layer 106 in order to form the local region 105.The absolute value of the initial break voltage may be higher than awriting voltage. The writing voltage is a voltage to be applied betweenthe first conductive layer 103 and the second conductive layer 106 sothat the resistive film 104 is reversibly changed between a highresistive state and a low resistive state. The absolute value of theinitial break voltage may be lower than the writing voltage. In thiscase, the initial break voltage may be repeatedly applied or may becontinuously applied for a predetermined time. By application of theinitial break voltage, as shown in FIG. 1A, the local region 105 incontact with the second conductive layer 106 and not in contact with thefirst conductive layer 103 is formed.

The local region 105 is a minute region corresponding to filamentsrequired for a current flow. The formation of filaments in the localregion 105 may be explained using a percolation model.

The percolation model is a model based on the theory in which a randomdistribution of oxygen defect sites in the local region 105 is assumed,and when the density of the oxygen defects sites or the like exceeds apredetermined threshold value, the probability of forming linkagebetween the oxygen defect sites is increased.

According to the percolation model, the filament is formed when aplurality of oxygen defect sites in the local region 105 are linked witheach other, and the change in resistance of the resistive film 104occurs when the oxygen defect sites in the local region 105 aregenerated and lost.

In this embodiment, the “oxygen defect” indicates that oxygen in thismetal oxide is deficient from the stoichiometric composition thereof.The “density of oxygen defect sites” corresponds to the degree of oxygendeficiency. That is, when the degree of oxygen deficiency is increased,the density of oxygen defect sites is also increased.

The local region 105 may be formed only at one place of the resistivefilm 104 of the gas sensor 100. The number of local regions 105 formedin the resistive film 104 may be confirmed, for example, by an electronbeam absorbed current (EBAC) analysis.

When the local region 105 is present in the resistive film 104, byapplication of a voltage between the first conductive layer 103 and thesecond conductive layer 106, a current in the resistive film 104 flowsconcentratedly through the local region 105.

The size of the local region 105 is small. Hence, for example, the localregion 105 generates heat by a current of approximately several tens ofmicroamperes which flows when the resistance is read, and by this heatgeneration, a significant increase in temperature occurs. When a currentof approximately several tens of microamperes flows, the powerconsumption thereby is less than 0.1 mW.

The second conductive layer 106 is formed of a metal (such as Pt) havinga catalyst function, and the local region 105 is in contact with thesecond conductive layer 106. By the structure as described above, thesecond conductive layer 106 is heated by the heat generation in thelocal region 105, and a hydrogen atom is efficiently dissociated from ahydrogen-containing gas.

When a hydrogen-containing gas is contained in a gas to be inspected, atthe second conductive layer 106, a hydrogen atom is dissociated from ahydrogen-containing gas, the hydrogen atom thus dissociated is bonded toan oxygen atom in the local region 105, and as a result, the resistanceof the local region 105 is decreased.

As described above, the gas sensor 100 has a characteristic in which theresistance between the first conductive layer 103 and the secondconductive layer 106 is decreased when the second conductive layer 106is in contact with a hydrogen-containing gas. By this characteristicdescribed above, when a gas to be inspected is brought into contact withthe second conductive layer 106, by detecting the decrease in resistancebetween the first conductive layer 103 and the second conductive layer106, a hydrogen-containing gas contained in the gas can be detected.

In addition, even if the local region 105 is placed in any one of a highresistive state and a low resistive state, when a hydrogen-containinggas is brought into contact with the second conductive layer 106, theresistance is further decreased. Hence, regardless of whether the localregion 105 is placed in any one of a high resistive state and a lowresistive state, the gas sensor 100 can detect hydrogen. However, inorder to more clearly detect the decrease in resistance, a gas sensor100 in which the local region 105 is set in an high resistive state inadvance may also be used.

Hereinafter, details of the gas sensor 100 configured to obtain a stableresistance change characteristic will be described.

The resistive film 104 is formed of an oxygen deficient-type metaloxide. A mother metal of the metal oxide may be at least one selectedfrom the group consisting of transition metals, such as tantalum (Ta),hafnium (Hf), titanium (Ti), zirconium (Zr), niobium (Nb), tungsten (W),nickel (Ni), and iron (Fe), and aluminum (Al). Since a transition metalis able to have a plurality of oxidized states, different resistivestates can be realized by an oxidation-reduction reaction. In this case,the oxygen deficient-type metal oxide is a metal oxide having a highdegree of oxygen deficiency as compared to a metal oxide which containsthe same metal as that thereof and which has a stoichiometriccomposition. A metal oxide having a stoichiometric composition is atypical insulating material, and on the other hand, an oxygendeficient-type metal oxide typically shows semiconductorcharacteristics. When an oxygen deficient-type metal oxide is used forthe resistive film 104, the gas sensor 100 is able to realize a stableresistance change operation with good reproducibility.

For example, when a hafnium oxide is used as a metal oxide forming theresistive film 104, and the composition thereof is represented byHfO_(x), if x is 1.6 or more, the resistance of the resistive film 104can be stably changed. In this case, the thickness of the hafnium oxidemay be set to 3 to 4 nm.

In addition, when a zirconium oxide is used as a metal oxide forming theresistive film 104, and the composition thereof is represented byZrO_(x), if x is 1.4 or more, the resistance of the resistive film 104can be stably changed. In this case, the thickness of the zirconiumoxide may be set to 1 to 5 nm.

In addition, when a tantalum oxide is used as a metal oxide forming theresistive film 104, and the composition thereof is represented byTaO_(x), if x is 2.1 or more, the resistance of the resistive film 104can be stably changed.

The compositions of the above respective metal oxide layers each can bemeasured using a Rutherford backscattering method.

As a material of the first conductive layer 103 and the secondconductive layer 106, for example, one selected from the groupconsisting of platinum (Pt), iridium (Ir), palladium (Pd), silver (Ag),nickel (Ni), tungsten (W), copper (Cu), aluminum (Al), tantalum (Ta),titanium (Ti), titanium nitride (TiN), tantalum nitride (TaN), andtitanium aluminum nitride (TiAlN) may be used.

In particular, the second conductive layer 106 may be formed, forexample, of a material having a catalyst function, such as platinum(Pt), iridium (Ir), or palladium (Pd), which dissociates a hydrogen atomfrom a gas molecule containing a hydrogen atom. In addition, the firstconductive layer 103 may be formed, for example, of a material having alow standard electrode potential, such as tungsten (W), nickel (Ni),tantalum (Ta), titanium (Ti), aluminum (Al), tantalum nitride (TaN), ortitanium nitride (TiN), as compared to that of a metal forming the metaloxide. As the standard electrode potential of a metal is higher, themetal is more unlikely to be oxidized.

In addition, as the substrate 101, for example, although a siliconsingle crystal substrate or a semiconductor substrate may be used, thesubstrate 101 is not limited thereto. Since the resistive film 104 canbe formed at a relatively low substrate temperature, for example, theresistive film 104 may be formed on a resin material or the like.

In addition, the gas sensor 100 may further include, for example, afixed resistance, a transistor, or a diode, as a load elementelectrically connected to the resistive film 104.

Modified Example 1

FIG. 2 is a cross-sectional view showing one structural example of a gassensor 200 according to a modified example 1 of the first embodiment.Hereinafter, among constituent elements of the gas sensor 200, the sameconstituent element as that of the gas sensor 100 of the firstembodiment is designated by the same reference numeral, the descriptionthereof is omitted, and a different point will only be described.

Since a second conductive layer 206 is a laminate including a lowerlayer 206 a in contact with the resistive film 104 and an upper layer206 b disposed on the lower layer 206 a, the gas sensor 200 is differentfrom the gas sensor 100 of the first embodiment. The other constituentelements of the gas sensor 200 are the same as those of the gas sensor100.

The lower layer 206 a is provided to have an approximately flat shapewith a thickness of an exposed first portion 2061. The upper layer 206 bis provided, other than the first portion 2061, on the lower layer 206 ain a second portion 2062 of the second conductive layer 206 which iscovered with the insulation layer 107.

Accordingly, the lower layer 206 a is exposed at the first portion 2061through the opening 108. The thickness of the first portion 2061 atwhich the second conductive layer 206 is exposed is smaller than thethickness of the second portion 2062 of the second conductive layer 206covered with the insulation layer 107.

In this case, the lower layer 206 a is formed of a material, such asplatinum or palladium, having a catalyst function. The upper layer 206 bis formed of an conductive material, such as titanium nitride (TiN).

By the structure as described above, the lower layer 206 a having acatalyst function can be designed to have a small thickness (such as 15nm). When titanium nitride used as the upper layer 206 b is etched toform the opening 108, platinum or palladium used as the lower layer 206a works as an etching stopper. As a result, by the lower layer 206 ahaving a catalyst function, the time for detecting hydrogen can beaccurately designed.

Modified Example 2

FIG. 3 is a cross-sectional view showing one structural example of a gassensor 300 according to a modified example 2 of the first embodiment.Hereinafter, among constituent elements of the gas sensor 300, the sameconstituent element as that of the gas sensor 100 according the firstembodiment or the gas sensor 200 according to the modified example 1 ofthe first embodiment is designated by the same reference numeral, thedescription thereof is omitted, and a different point will only bedescribed.

As is the gas sensor 200, a second conductive layer 306 of the gassensor 300 is a laminate formed of a lower layer 306 a connected to theresistive film 104 and an upper layer 306 b disposed on the lower layer306 a.

The lower layer 306 a is provided to have an approximately flat shapewith a thickness of a first portion 3061 at which the second conductivelayer 306 is exposed. The upper layer 306 b is provided, other than thefirst portion 3061, on the lower layer 306 a and the insulation layer107 in a second portion 3062 of the second conductive layer 306 which iscovered with the insulation layer 107.

In the gas sensor 300, the lower layer 306 a is provided to have thesame size (the same shape when viewed in plan) as that of the resistivefilm 104, and this is a point different from the gas sensor 200.

By this structure, the lower layer 306 a and the resistive film 104 canbe formed by one etching step. Hence, the interface between the lowerlayer 306 a and the resistive film 104 can be protected from intrusionof foreign substances caused by a process. As a result, the catalystfunction of the lower layer 306 a can be stably obtained, and a gassensor excellent in detection of a hydrogen-containing gas can beobtained.

Modified Example 3

FIG. 4A is a cross-sectional view showing a structural example of a gassensor 400 according to a modified example 3 of the first embodiment.Hereinafter, among constituent elements of the gas sensor 400, the sameconstituent element as that of the gas sensor 100 according the firstembodiment, the gas sensor 200 according to the modified example 1 ofthe first embodiment, or the gas sensor 300 according to the modifiedexample 2 of the first embodiment is designated by the same referencenumeral, the description thereof is omitted, and a different point willonly be described.

In the gas sensor 400, various conductive members to be used forelectrical connection are additionally formed in the gas sensor 300. Inparticular, the gas sensor 400 includes the same constituent elements asthose of the gas sensor 300 at important portions and further includes afirst wire 414, a second wire 416, a third wire 411, and contact plugs409, 413, and 415.

The contact plug 413 and the first wire 414 are provided in theinsulation layer 102. The contact plug 413 is connected to the firstconductive layer 103 and the first wire 414. The contact plug 415 isprovided in the insulation layer 102 and the insulation layer 107 and isconnected to the first wire 414 and the second wire 416. The contactplug 409 is provided in the insulation layer 107 and is connected to theupper layer 306 b and the third wire 411.

By the structure described above, when a detection voltage is appliedbetween the second wire 416 and the third wire 411, a detection resultof a hydrogen-containing gas can be obtained from the gas sensor 400.

In addition, besides the contact plug 409, the connection between theupper layer 306 b and the third wire 411 may be obtained by a differentstructure. For example, as is the connection between the firstconductive layer 103 and the second wire 416, the connection may beperformed by two contact plugs and one wire provided in the insulationlayer 102. In this case, even if the third wire 411 is not providedabove the upper layer 306 b, the upper layer 306 b and the third wire411 can be connected to each other.

[Supplement]

The gas sensor 100 shown in FIG. 1A includes the first conductive layer103, the metal oxide layer 104 disposed on the first conductive layer103, the second conductive layer 106 disposed on the metal oxide layer104, and the insulation layer 107 which covers the layers describedabove.

In FIG. 1A, the second conductive layer 106 is formed of a singlematerial. The second conductive layer 106 includes a first region 1061(that is, the first portion 1061) and a second region 1062 (that is, thesecond portion 1062). The first region 1061 is a region having arelatively small thickness, and the second region 1062 is a regionhaving a relatively large thickness. In the plan view shown in FIG. 1B,the first region 1061 is surrounded by the second region 1062. The uppersurface of the second conductive layer 106 has a concave portion, andthe lower surface of the second conductive layer 106 is flat. The uppersurface of the first region 1061 is exposed to a gas which is to bedetected.

The metal oxide layer 104 includes the local region 105 and the bulkregion surrounding the local region 105. In this case, “surroundings thelocal region 105” is not limited to the case in which all the outerperipheral surface of the local region 105 is surrounded. In FIG. 1A,the bulk region is a region of the metal oxide layer 104 other than thelocal region 105. The degree of oxygen deficiency of the local region105 is high as compared to that of the bulk region. In the plan viewshown in FIG. 1B, the outline of the metal oxide layer 104 is locatedinside the outline of the second conductive layer 106. In FIG. 1A, themetal oxide layer 104 has a flat surface in contact with the firstregion 1061 and the second region 1062 of the second conductive layer106.

The insulation layer 107 covers the first conductive layer 103, themetal oxide layer 104, and the second region 1062 of the secondconductive layer 106. The insulation layer 107 does not cover the firstregion 1061 of the second conductive layer 106. In the example shown inFIGS. 1A and 1B, the insulation layer 107 has the opening 108 whichreaches the first region 1061 of the second conductive layer 106.

The gas sensor 400 shown in FIG. 4A includes the first conductive layer103, the metal oxide layer 104 disposed on the first conductive layer103, the second conductive layer 306 disposed on the metal oxide layer104, and the insulation layer 107 covering the layers described above.

In FIG. 4A, the second conductive layer 306 includes a first layer 306 a(that is, the lower layer 306 a) having a flat shape and a second layer306 b (that is, the upper layer 306 b) disposed partially on the firstlayer. The boundary surface between the first layer 306 a and the secondlayer 306 b is approximately in parallel to the lower surface of thesecond conductive layer 306.

In addition, the second conductive layer 306 includes a first region, asecond region, and a third region. The boundary surfaces between thefirst region, the second region, and the third region are eachapproximately perpendicular to the lower surface of the secondconductive layer 306. In FIG. 4A, the first portion 3061 corresponds tothe first region. In the second region of the second conductive layer306, the second layer 306 b is disposed on the first layer 306 a. InFIG. 4A, in the second portion 3062, a region in which the first layer306 a and the second layer 306 b are laminated to each other correspondsto the second region. In the third region of the second conductive layer306, the first layer 306 a is not disposed under the second layer 306 b.In FIG. 4A, in the second portion 3062, a region in which the secondlayer 306 b extends past the first layer 306 a corresponds to the thirdregion. The first region is a region having a relatively smallthickness, and the second region is a region having a relatively largethickness. In the plan view shown in FIG. 4B, the first region issurrounded by the second region. In FIG. 4B, the second layer 306 b hasthe opening which reaches the upper surface of the first layer 306 a.The upper surface of the second conductive layer 306 has a concaveportion, and the lower surface of the second conductive layer 306 isflat. The bottom surface of the concave portion is defined by a part ofthe upper surface of the first layer 306 a, and the side surface of theconcave portion is defined by the inner peripheral surface of the secondlayer 306 b. In the plan view shown in FIG. 4B, the outline of the firstlayer 306 a is located inside the outline of the second layer 306 b.

In the example shown in FIG. 4A, the gas sensor 400 includes the plug409 penetrating the insulation layer 107 and the wire 411 provided onthe insulation layer 107 and the plug 409. The plug 409 electricallyconnects the third region of the second conductive layer 306 to the wire411.

[Manufacturing Method and Operation of Gas Sensor]

Next, with reference to FIGS. 5A to 5I, one example of a manufacturingmethod of the gas sensor 400 will be described. In addition, thefollowing manufacturing method is not only applied to the manufacturingof the gas sensor 400 but may also be applied to the manufacturing ofthe gas sensor 100, 200, or 300 after being partially appropriatelymodified.

First, as shown in FIG. 5A, in a step of forming the first wire 414, onthe substrate 101 on which a transistor, an underlayer wire, and thelike are formed, the insulation layer 102 is formed. In the firstinsulation layer 102, an conductive layer formed of aluminum or the likehaving a thickness of 400 to 600 nm is formed and is then patterned, sothat the first wire 414 is formed. In addition, in a step of forming thecontact plug 413, patterning is performed using a desired mask, so thatthe contact plug 413 connected to the first wire 414 is formed in theinsulation layer 102.

In addition, on the insulation layer 102, an conductive layer 103′ to beformed into the first conductive layer 103 is formed. As the conductivelayer 103′, for example, a TaN thin film having a thickness of 100 nmmay be formed by a sputtering method. In addition, between theconductive layer 103′ and the insulation layer 102, an adhesive layermay be formed from Ti, TiN, or the like by a sputtering method.

Subsequently, on the conductive layer 103′, a metal oxide layer 104′ tobe formed into the resistive film 104 is formed. As the metal oxidelayer 104′, for example, an oxygen deficient-type tantalum oxide layermay be formed by a reactive sputtering method using a Ta target or thelike. The tantalum oxide layer has a high initial resistance when thethickness thereof is excessively large, and when the thickness isexcessively small, a stable resistance change may not be obtained; hencethe thickness may be set in a range of 1 to 8 nm.

Next, on the metal oxide layer 104′, an conductive layer 306 a′ to beformed into the lower layer 306 a is formed. As the conductive layer 306a′, for example, a Pt thin film having a thickness of 15 nm may beformed by a sputtering method.

Next, as shown in FIG. 5B, by a photolithographic step, a mask 420 isformed above the contact plug 413 using a photoresist.

Next, as shown in FIG. 5C, by dry etching using the mask 420, theconductive layer 103′, the metal oxide layer 104′, and the conductivelayer 306 a′ are formed to have an element shape, so that the firstconductive layer 103, the resistive film 104, and the lower layer 306 aare formed.

Subsequently, as shown in FIG. 5D, an insulation layer 107 a isdeposited on the insulation layer 102. Next, by chemical mechanicalpolishing (CMP), the insulation layer 107 a is etched back so as toexpose the lower layer 306 a. On the insulation layer 107 a, anconductive layer 306 b′ to be formed into the upper layer 306 b isformed so as to be in contact with the lower layer 306 a. As theconductive layer 306 b′, for example, a TiN thin film having a thicknessof 150 nm may be formed by a sputtering method.

Next, as shown in FIG. 5E, by a photolithographic step, a mask 421 isformed on the conductive layer 306 b′ using a photoresist in a regionincluding a position located above the lower layer 306 a.

Subsequently, as shown in FIG. 5F, by dry etching using the mask 421,the conductive layer 306 b′ is formed into the upper layer 306 b.

Next, as shown in FIG. 5G, on the insulation layer 107 a and the upperlayer 306 b, an insulation layer 107 b is deposited. Subsequently, byusing CMP, the insulation layer 107 b is etched back until the upperlayer 306 b is exposed.

Next, as shown in FIG. 5H, on the insulation layer 107 b and the upperlayer 306 b, an insulation layer 107 c is deposited. A via(through-hole) which reaches the upper surface of the upper layer 306 bis formed in the insulation layer 107 c, and an conductive material isfilled in the via to form the contact plug 409. In addition, a via whichreaches the upper surface of the first wire 414 is formed through theinsulation layers 107 a to 107 c, and an conductive material is filledin this via to form the contact plug 415. Furthermore, a new conductivefilm is disposed on the insulation layer 107 c and is then patterned, sothat the third wire 411 connected to the contact plug 409 and the secondwire 416 connected to the contact plug 415 are formed.

Subsequently, as shown in FIG. 5I, by performing etching, parts of theinsulation layer 107 c and the upper layer 306 b located above the lowerlayer 306 a are removed, so that the opening 108 through which the uppersurface of the lower layer 306 a is partially exposed is formed.

Next, by applying an initial break voltage between the second wire 416and the third wire 411, the local region 105 is formed in the resistivefilm 104, so that the gas sensor 400 shown in FIG. 4A is formed.

Hereinafter, as for one example of the resistance change characteristicof the gas sensor 100 by voltage application, actual measurement resultsobtained by a sample element will be described. In addition, theresistance change characteristic of the gas sensor 100 by ahydrogen-containing gas will be described later.

FIG. 6 is a graph showing a resistance change characteristic actuallymeasured using a sample element.

In the gas sensor 400 which is a sample element, the measurement resultsof which are obtained as shown in FIG. 6, the size of each of the firstconductive layer 103, the lower layer 306 a, and the resistive film 104is set to 0.5 μm by 0.5 μm (area: 0.25 μm²). In addition, when thecomposition of a tantalum oxide used as the resistive film 104 isrepresented by TaO_(y), y is set to 2.47. Furthermore, the thickness ofthe resistive film 104 is set to 5 nm. By using the gas sensor 400 asdescribed above, when a reading voltage (such as 0.4 V) is appliedbetween the first conductive layer 103 and the lower layer 306 a, aninitial resistance RI is approximately 10⁷ to 10⁸Ω.

As shown in FIG. 6, when the resistance of the gas sensor 400 is theinitial resistance RI (higher than a resistance HR in a high resistivestate), by applying the initial break voltage between the firstconductive layer 103 and the lower layer 306 a, the resistive state ischanged. Subsequently, as a writing voltage, for example, when two typesof voltage pulses (a positive voltage pulse and a negative voltagepulse) having a pulse width of 100 ns and different polarities arealternately applied between the first conductive layer 103 and the lowerlayer 306 a of the gas sensor 400, the resistance of the resistive film104 is reversibly changed.

That is, as a writing voltage, when a positive voltage pulse (pulsewidth: 100 ns) is applied between the conductive layers, the resistanceof the resistive film 104 is increased from a low resistance LR to thehigh resistance HR. On the other hand, as a writing voltage, when anegative voltage pulse (pulse width: 100 ns) is applied between theconductive layers, the resistance of the resistive film 104 is decreasedfrom the high resistance HR to the low resistance LR. In addition, asfor the polarity of the voltage pulse, when the potential of the upperlayer 306 b is high as compared to that of the first conductive layer103, the polarity is “positive”, and when the potential of the lowerlayer 306 a is low as compared to that of the first conductive layer103, the polarity is “negative”.

By the use of the resistance change characteristic obtained by thevoltage application as described above, before monitoring of ahydrogen-containing gas is started, when a positive voltage pulse isapplied between the first conductive layer 103 and the lower layer 306a, a hydrogen-containing gas can be detected using the gas sensor 400which is set in the high resistive state (HR). Accordingly, compared tothe case in which a hydrogen-containing gas is detected using the gassensor 400 which is set in the low resistive state (LR), the decrease inresistance can be more clearly detected; hence, detection performance ofa hydrogen-containing gas is improved.

Modified Example 4

FIG. 7 is a cross-sectional view showing one structural example of a gassensor 500 according to a modified example 4 of the first embodiment.Hereinafter, among constituent elements of the gas sensor 500, the sameconstituent element as that of the gas sensor 100 according the firstembodiment, the gas sensor 200 according to the modified example 1 ofthe first embodiment, the gas sensor 300 according to the modifiedexample 2 of the first embodiment, or the gas sensor 400 according tothe modified example 3 of the first embodiment is designated by the samereference numeral, the description thereof is omitted, and a differentpoint will only be described.

The gas sensor 500 of this modified example 4 is different from the gassensor 400 since a resistive film 504 is a laminate formed of a firstoxide layer 504 a in contact with the first conductive layer 103 and asecond oxide layer 504 b in contact with the lower layer 306 a. Inaddition, the resistive film 504 may be a laminate containing not onlytwo layers but also at least three layers.

In the first oxide layer 504 a and the second oxide layer 504 b, thelocal region 105 is provided in which in response to the application ofan electrical pulse and a hydrogen-containing gas, the degree of oxygendeficiency is reversibly changed. The local region 105 is formed topenetrate at least the second oxide layer 504 b and to be in contactwith the lower layer 306 a.

In other words, the resistive film 504 has a laminate structure at leastcontaining a first metal oxide layer 504 a containing a first metaloxide and a second metal oxide layer 504 b containing a second metaloxide. In addition, the first metal oxide layer 504 a is disposedbetween the first conductive layer 103 and the second metal oxide layer504 b, and the second metal oxide layer 504 b is disposed between thefirst metal oxide layer 504 a and the lower layer 306 a.

The thickness of the second metal oxide layer 504 b may be smaller thanthat of the first metal oxide layer 504 a. In this case, the localregion 105 can be easily formed so as not to be in contact with thefirst conductive layer 103. The degree of oxygen deficiency of thesecond metal oxide layer 504 b may be low as compared to that of thefirst metal oxide layer 504 a. In this case, since the resistance of thesecond metal oxide layer 504 b is higher than that of the first metaloxide layer 504 a, the voltage applied to the resistive film 504 ismostly applied to the second metal oxide layer 504 b. By the structuredescribed above, for example, the initial break voltage can beconcentrated to the second metal oxide layer 504 b, and hence, theinitial break voltage required to form the local region 105 can beeffectively decreased.

In addition, in the present disclosure, when the metal forming the firstmetal oxide layer 504 a is the same as that forming the second metaloxide layer 504 b, instead of using the “degree of oxygen deficiency”,the term “oxygen content” may be used in some cases. The “high oxygencontent” corresponds to the “low degree of oxygen deficiency”, and the“low oxygen content” corresponds to the “high degree of oxygendeficiency”.

However, as described below, the resistive film 504 according to thisembodiment is not limited to the case in which the metal forming thefirst metal oxide layer 504 a is the same as that forming the secondmetal oxide layer 504 b, and the metal of the metal oxide layer 504 amay be different from the metal of the second metal oxide layer 504 b.That is, the first metal oxide layer 504 a and the second metal oxidelayer 504 b may be formed from oxides containing different metals fromeach other.

When a first metal forming the first metal oxide layer 504 a is the sameas a second metal forming the second metal oxide layer 504 b, the oxygencontent inversely corresponds to the degree of oxygen deficiency. Thatis, when the oxygen content of the second metal oxide is higher thanthat of the first metal oxide, the degree of oxygen deficiency of thesecond metal oxide is lower than that of the first metal oxide.

The resistive film 504 includes the local region 105 in the vicinity ofthe interface between the first metal oxide layer 504 a and the secondmetal oxide layer 504 b. The degree of oxygen deficiency of the localregion 105 is high as compared to that of the second metal oxide layer504 b and is different from the degree of oxygen deficiency of the firstmetal oxide layer 504 a.

By application of the initial break voltage between the first conductivelayer 103 and the lower layer 306 a, the local region 105 is formed inthe resistive film 504 having a laminate structure of the first metaloxide layer 504 a and the second metal oxide layer 504 b. By the initialbreak voltage, there can be formed the local region 105 which is incontact with the lower layer 306 a, which penetrates the second metaloxide layer 504 b and partially enters the first metal oxide layer 504a, and which is not in contact with the first conductive layer 103.

Modified Example 5

FIG. 8A is a cross-sectional view showing one structural example of agas sensor 600 according to a modified example 5 of the firstembodiment. FIG. 8B is a plan view showing one structural example of thegas sensor 600 according to the modified example 5 of the firstembodiment. The cross-section of FIG. 8A corresponds to a cross-sectionviewed in an arrow direction along the section line VIIIA-VIIIA of FIG.8B. Hereinafter, among constituent elements of the gas sensor 600, thesame constituent element as that of the gas sensor 100 according thefirst embodiment, the gas sensor 200 according to the modified example 1of the first embodiment, the gas sensor 300 according to the modifiedexample 2 of the first embodiment, the gas sensor 400 according to themodified example 3 of the first embodiment, or the gas sensor 500according to the modified example 4 of the first embodiment isdesignated by the same reference numeral, the description thereof isomitted, and a different point will only be described.

Since having an oxide film 617 along the periphery of the resistive film504, the gas sensor 600 according to this modified example 5 isdifferent from the gas sensor 500. The oxide film 617 is formed, forexample, by oxidizing the first metal oxide layer 504 a and the secondmetal oxide layer 504 b from the side surfaces thereof and has a higherresistance than that of the first metal oxide layer 504 a. In addition,instead of using the oxide film 617, an oxynitride film may also beformed by nitriding the first metal oxide layer 504 a and the secondmetal oxide layer 504 b from the side surfaces thereof.

Since the oxide film 617 having a high resistance is formed on the sidesurface portion of the resistive film 504, a contact area at which thefirst metal oxide layer 504 a and the second metal oxide layer 504 b,each of which has a low resistance, are in contact with each otherbecomes smaller than the area of the lower layer 306 a. The contact areadescribed above may be either smaller or larger than that of the opening108.

By the structure described above, the current density of a currentflowing from the first metal oxide layer 504 a to the second metal oxidelayer 504 b is increased. As a result, the initial break voltage of thegas sensor is decreased, and the initial break at a low voltage can berealized. Furthermore, since the local region 105 is formed in theregion of the opening 108, the time required for hydrogen contained in agas to be inspected to reach the local region 105 can be shortened.

As for the gas sensor 600 formed as described above, one evaluationexample of the resistance change characteristic by a hydrogen-containinggas will be described.

FIG. 9A is a block diagram showing one example of an evaluation systemused for the evaluation of the gas sensor 600. An evaluation system 900shown in FIG. 9A includes an air-tight container 910 receiving the gassensor 600, a power source 920, and a current meter 930. The air-tightcontainer 910 is connected to a hydrogen cylinder 911 and a nitrogencylinder 912 through introduction valves 913 and 914, respectively, andis also configured so that a gas in the container 910 is dischargeablethrough an exhaust valve 915.

FIG. 9B is a graph showing one evaluation result of the gas sensor 600.The horizontal axis indicates the time (a.u.), and the vertical axisindicates a current (a.u.) flowing between the first conductive layer103 and the lower layer 306 a. In this experiment, first, a nitrogen gaswas charged into the air-tight container 910 in which the gas sensor 600was placed, and a hydrogen gas was then charged.

FIG. 9B shows the result of this evaluation, and along the horizontalaxis, two types of periods, that is, the period for nitrogenintroduction and the period of hydrogen introduction, are shown. It isfound that after the introduction gas was switched from a nitrogen gasto a hydrogen gas, the current was started to increase.

In this evaluation example, a predetermined voltage (potentialdifference) was applied between the first conductive layer 103 and thelower layer 306 a, and as a result, the local region 105 was set inadvance in a high resistive state. In operation of monitoring ahydrogen-containing gas, a detection voltage of 0.6 V was appliedbetween the first conductive layer 103 and the lower layer 306 a. In thestate in which a hydrogen gas is detected, a current of 10 to 200 μAflowed between the first conductive layer 103 and the lower layer 306 a.Hence, it is found that by the gas sensor 600, a hydrogen-containing gascan be monitored by a significantly small power consumption of at most0.006 to 0.12 mW.

In addition, when a detection voltage of 0.4 V was applied between thefirst conductive layer 103 and the lower layer 306 a, the resistance wasnot changed by a hydrogen gas, and a hydrogen gas could not be detected.The reason for this is believed that the amount of heat generated in thelocal region 105 is not sufficient by the application of a detectionvoltage of 0.4 V, and the catalyst function of the lower layer 306 a isnot sufficiently facilitated. It is believed that for detection of ahydrogen gas, for example, a detection voltage of 0.6 V or more isrequired to be applied.

From the results described above, the detection mechanism of a hydrogengas by the gas sensor 600 can be considered as follows.

When a hydrogen-containing gas is brought into contact with the secondconductive layer 306 (in particular, the lower layer 306 a), by thecatalyst function of the lower layer 306 a, a hydrogen atom isdissociated from a hydrogen-containing gas. In order to maintain theequilibrium, the hydrogen atom thus dissociated diffuses in the lowerlayer 306 a and reaches the local region 105.

By this hydrogen atom, a reduction reaction occurs in the minute localregion 105, and the degree of oxygen deficiency in the local region 105is increased. As a result, filaments in the local region 105 are likelyto be linked with each other, so that the resistance of the local region105 is decreased. As a result, it is believed that a current flowingbetween the first conductive layer 103 and the lower layer 306 a isincreased.

In addition, it is also believed that the operation described above isperformed not only in the gas sensor 600 but also in the gas sensors100, 200, 300, 400 and 500, the important portion of each of which hassubstantially the same structure as that of the gas sensor 600. Inaddition, it is also believed that a detectable gas is not limited to ahydrogen gas, and that the operation described above may also beperformed for various types of hydrogen-containing gases, such asmethane and an alcohol.

As described above, according to the gas sensor 600 of this embodiment,a sensor excellent in electrical power saving can be obtained in whichheat generation is performed only by a current used for detecting theresistive state, and without performing heating by an additional heater,a hydrogen-containing gas can be detected.

[Dependence of Detection Time of Hydrogen-Containing Gas on Thickness ofSecond Conductive Layer]

The dependence of a time required for detection of a hydrogen-containinggas of a gas sensor on the thickness of the second conductive layer willbe described based on the mechanism in which the gas sensor detects ahydrogen-containing gas. In addition, in order to facilitate theunderstanding, by the use of a gas sensor 700 shown in FIG. 10A in whicha resistive film 704 is formed of a single layer, the mechanism will bedescribed; however, the following description may also be applied to agas sensor in which the resistive film 704 is formed from a plurality oflayers.

FIG. 10A is a schematic view showing the resistive state and the oxygendefect state of the gas sensor 700. FIG. 10A shows the fundamental gassensor 700 formed of a first conductive layer 703, the resistive film704, a local region 705, and a second conductive layer 706.

As schematically shown in FIG. 10A, it is estimated from the evaluationresult obtained by the simulation performed by the present inventor thatin the local region 705 of the gas sensor 700 which is maintained in ahigh resistive state having an average resistance of approximately 40kΩ, approximately 7,870 oxygen defects 710 are present. In the statedescribed above, the number of the oxygen defects 710 in the localregion 705 is not sufficient to form the filament, and the gas sensor700 is maintained in a high resistive state.

When the number of the oxygen defects 710 present in the local region705 in a low resistive state is evaluated by the simulation, the lowresistive state being obtained from the above high resistive state insuch a way that a hydrogen gas (hydrogen atoms) is allowed to reach thelocal region 705 after passing through the second conductive layer 706so that the resistance is decreased by approximately one digit, it isestimated that the number of the oxygen defects 710 is increased toapproximately 10,090. When a hydrogen atom reaches the local region 705,this hydrogen atom reacts with oxygen in the local region 705, and as aresult, a new oxygen defect 711 is generated. When the oxygen defect 711and the existing oxygen defect 710 are linked with each other, afilament is formed, and the gas sensor 700 is changed into a lowresistive state.

Accordingly, in order to decrease the resistance of the gas sensor 700by approximately one digit, the number (such as 10,090−7,870=2,220) ofhydrogen molecules approximately equivalent to the number of increasedoxygen defects is required to reach the local region 705.

FIG. 10B is a graph showing the relationship between the resistance ofthe gas sensor 700 and the number of oxygen defects present in the localregion 705.

The number of hydrogen molecules which reach the local region 705 can becalculated by the following Equation 1 in consideration of the diffusionof hydrogen molecules in the second conductive layer 706. In addition,in Equation 1, the case in which the second conductive layer 706 isformed from platinum (Pt) is assumed.

$\begin{matrix}{{n = {N_{0}p\; A\sqrt{\frac{N_{A}\kappa_{B}T}{2\Pi\; M}}{{erfc}\left( \frac{x}{2\sqrt{D_{Pt}t}} \right)}}}{\text{x}\text{: thickness of second conductive layer,}\text{t}\text{: time,}}{\text{p}\text{: hydrogen concentration,}\text{A}\text{: area of local region}}} & {{Equation}\mspace{14mu} 1}\end{matrix}$Constants at 1 atm and 25° C. when second conductive layer is formed ofPt

Parameter (1 atm 25° C.) Value Unit Number of gas molecules per No 2.46× 10¹⁹ cm⁻³ unit volume Molecular weight of hydrogen M 2.02 gmol⁻¹Diffusion coefficient of D_(pt)    1.45 × 10¹² √{square root over(P_(H2))} cm⁻²s⁻¹ hydrogen in Pt [1], [2] Avogadro constant N_(A) 6.02 ×10²³ Boltzmann constant K_(B)  1.38 × 10⁻²³ JK⁻¹ [1] S. Uemiya, Topicsin Catalysis 29, 79, 2004 [2] J. D. Fast Interaction of Metals andGasses 1965

In accordance with Equation 1, the number of hydrogen molecules per unittime which reaches the local region 705 by diffusion through the secondconductive layer 706 is influenced by a number N₀ of hydrogen moleculescontained in a gas to be inspected in contact with the second conductivelayer 706 and a thickness x of the second conductive layer 706.

FIG. 11 is a graph showing the relationship between an exposure time ofthe gas sensor 700 to a hydrogen-containing gas and the number ofhydrogen molecules which reach the local region 705. In the case inwhich, for example, the thickness of the second conductive layer 706formed from platinum is set to 18.6 nm, the exposure time dependence ofthe number (hereinafter, referred to as N(H₂)) of hydrogen moleculeswhich reach the local region 705 is shown. In FIG. 11, the exposure timedependences at four concentrations, that is, 0.1%, 1%, 10%, and 100%,are shown. From FIG. 11, it is found that when a critical timecorresponding to each hydrogen concentration passes, N(H₂) is rapidlyincreased. In addition, as the concentration of hydrogen in a gas to bemeasured is increased, the rise time of N(H₂) is shortened.

FIG. 11 shows that when the concentration of hydrogen in a gas to beinspected is high, that is, when the number N₀ of hydrogen molecules islarge, the number of hydrogen molecules required to form filamentsrapidly reaches the local region 705, and hence the gas sensor 700 ischanged into a low resistive state in a short time.

As one example, FIG. 12 shows a calculation result showing therelationship between the thickness of the second conductive layer 706and the concentration of a hydrogen gas at which the number (such as2,220) of hydrogen molecules required to form filaments reaches thelocal region 705 after one second from the introduction of a hydrogengas (that is, exposure to a hydrogen gas is performed for one second).

FIG. 12 shows that even if the concentration of hydrogen in a gas to beinspected is low, when the thickness of the second conductive layer 706is small, the number (such as 2,220) of hydrogen molecules required toform filaments reaches the local region within one second.

Hence, when the second conductive layer 706 is formed to have athickness at which the number of hydrogen molecules required for theresistance change of the resistive film 704 passes therethrough for apredetermined time, a hydrogen gas at a desired concentration can bedetected within a predetermined time.

As described above, by the use of the gas sensors 100 to 600 describedin the embodiment and the modified examples, when the second conductivelayer 106 or the lower layer 206 a or 306 a is formed to have a smallthickness, a gas sensor excellent in detection of a hydrogen-containinggas can be obtained in which the detection time of a hydrogen-containinggas can be shortened, and a hydrogen-containing gas at a lowerconcentration can be detected.

Second Embodiment

A fuel-cell vehicle according to a second embodiment comprises one ofthe gas sensors described in the first embodiment and the modifiedexamples as a hydrogen sensor. This fuel-cell vehicle is a fuel-cellvehicle detecting a hydrogen gas in the vehicle using the hydrogensensor.

FIG. 13 is a side view showing one structural example of a fuel-cellvehicle 800 according to the second embodiment.

The fuel-cell vehicle 800 includes a passenger compartment 810, abaggage room 820, a gas tank room 830, a fuel tank 831, a hydrogensensor 832, a pipe arrangement 840, a fuel-cell room 850, a fuel cell851, a hydrogen sensor 852, a motor room 860, and a motor 861.

The fuel tank 831 is provided in the gas tank room 830 and contains ahydrogen gas as a fuel gas. The hydrogen sensor 832 detects a fuel gasleakage in the gas tank room 830.

The fuel cell 851 is formed as a fuel-cell stack in which cells eachincluding a fuel electrode, an air electrode, and an electrolyte as abasic unit are stacked to each other. This fuel cell 851 is provided inthe fuel cell room 850. A hydrogen gas in the fuel tank 831 is suppliedto the fuel cell 851 in the fuel-cell room 850 through the pipearrangement 840. The fuel cell 851 generates an electrical power by areaction between this hydrogen gas and an oxygen gas in the air. Thehydrogen sensor 852 detects a hydrogen gas leakage in the fuel-cell room850.

The motor 861 is provided in the motor room 860 and is rotated by anelectrical power generated by the fuel cell 851, so that the fuel-cellvehicle 800 is driven.

As described above, in the gas sensor according to the presentdisclosure, as one example, by a very small power consumption ofapproximately 0.01 mW, a hydrogen gas can be detected. Hence, when thegas sensor is used for the hydrogen sensors 832 and 852, by excellentelectrical power saving performance thereof, the hydrogen gas leakagecan be always monitored without remarkably increasing a standbyelectrical power of the fuel-cell vehicle.

For example, regardless of the operation state of an ignition key of thefuel-cell vehicle 800, a predetermined voltage may be always applied tothe hydrogen sensors 832 and 852. In this case, based on a currentflowing through the hydrogen sensors 832 and 852, the presence orabsence of a hydrogen gas outside the fuel tank 831 in the gas tank 830and outside the fuel cell 851 in the fuel cell room 850 may be judged.

Accordingly, for example, when the ignition key is operated, since thepresence or absence of a hydrogen gas leakage is already judged,compared to the case in which the presence or absence of a hydrogen gasleakage is judged after the ignition key is operated, the start-up timeof a fuel-cell vehicle can be shortened. In addition, for example, evenafter the fuel-cell vehicle is driven and then stored in a garage, thesafety can be improved by continuously monitoring a hydrogen gasleakage.

Overview of Embodiments

A gas sensor according to one aspect comprises a first conductive layerand a second conductive layer, the respective principal surfaces ofwhich face each other; a metal oxide layer in contact with the principalsurface of the first conductive layer and the principal surface of thesecond conductive layer; a local region which is disposed in the metaloxide layer so as to be in contact with the second conductive layer andwhich has a high degree of oxygen deficiency as compared to that of themetal oxide layer; and an insulation layer covering the first conductivelayer, the second conductive layer, and the metal oxide layer. The otherprincipal surface of the second conductive layer facing the aboveprincipal surface thereof is at least partially exposed without beingcovered with the insulation layer; the thickness of a first portionwhich is the exposed portion of the second conductive layer is smallerthan the thickness of a second portion which is a portion of the secondconductive layer covered with the insulation layer; and when the secondconductive layer is in contact with a gas containing a gas moleculehaving a hydrogen atom, the resistance between the first conductivelayer and the second conductive layer is decreased.

According to the structure as described above, a current flowing betweenthe first conductive layer and the second conductive layer isconcentrated in the local region having a high degree of oxygendeficiency. As a result, by a small current, the temperature of thelocal region can be increased. Hence, by the use of the self-heating andthe gas sensitivity in the local region formed in the metal oxide layer,a hydrogen sensor excellent in electrical power saving can be obtainedwhich is able to detect a hydrogen-containing gas without performingheating by a heater.

Since the local region is heated by a current flowing between the firstconductive layer and the second conductive layer, at a portion of thesecond conductive layer in contact with the local region, a hydrogenatom is dissociated from the hydrogen-containing gas, and the hydrogenatom thus dissociated is bonded to an oxygen atom in the local region ofthe metal oxide layer, so that the resistance between the firstconductive layer and the second conductive layer is decreased.

In more particular, when the temperature of the local region isincreased, the temperature of the surface of the second conductive layeris also increased. In accordance with the increase in temperature, bythe catalyst function of the second conductive layer, the efficiency ofdissociation of a hydrogen atom from a gas molecule having a hydrogenatom at the second conductive layer is improved.

When a gas molecular having a hydrogen atom is brought into contact withthe second conductive layer, a hydrogen atom is dissociated from the gasmolecule, and the hydrogen atom thus dissociated diffuses in the secondconductive layer and reaches the local region. In addition, the hydrogenatom is bonded to oxygen of a metal oxide present in the local region toform water (H₂O), and as a result, the degree of oxygen deficiency ofthe local region is further increased. Accordingly, a current is likelyto flow in the local region, and the resistance between the firstconductive layer and the second conductive layer is decreased.

Since the first portion having a thickness smaller than the thickness ofthe second portion is provided in the second conductive layer, thehydrogen atom dissociated from the hydrogen-containing gas at the firstportion rapidly reaches the metal oxide layer. Accordingly, theresistance between the first conductive layer and the second conductivelayer is rapidly decreased, and a hydrogen-containing gas can be rapidlydetected.

In addition, the second conductive layer may be formed of a single layerin which the thickness of the first portion is different from that ofthe second portion.

According to the structure as described above, for example, by arelatively simple method in which after a flat plate having thethickness of the second conductive layer is formed, a part thereof isremoved to form the first portion, the above second conductive layer isobtained.

In addition, the second conductive layer may be formed of anapproximately flat-shaped first layer having the thickness of the firstportion and a second layer provided on the first layer except for thefirst portion.

According to the structure as described above, for example, by a methodin which after the first layer and the second layer are laminated toeach other so as to have respective predetermined thicknesses, thesecond layer is selectively removed at the first portion, the secondconductive layer in which the thickness of the first portion isaccurately controlled can be obtained.

In addition, the gas sensor may further include a via which penetratesthe insulation layer and is connected to the second portion of thesecond conductive layer.

According to the structure as described above, since the via isconnected to the second portion having a thickness larger than that ofthe first portion of the second conductive layer, as compared to thecase in which the via is connected to the first portion, the reliabilityof the electrical connection between the via and the second conductivelayer is improved.

In addition, the metal oxide layer is formed by laminating a first metaloxide layer formed of a first metal oxide and a second metal oxide layerformed of a second metal oxide having a low degree of oxygen deficiencyas compared to that of the first metal oxide; the first metal oxidelayer is in contact with the first conductive layer, and the secondmetal oxide layer is in contact with the second conductive layer; thelocal region is formed to penetrate at least the second metal oxidelayer and to be in contact with the second conductive layer and may havea high degree of oxygen deficiency as compared to that of the secondmetal oxide layer.

According to the structure as described above, a laminate structureexcellent in resistance change characteristic is used for the metaloxide layer, and hence, a gas sensor excellent in detection of ahydrogen-containing gas can be obtained.

In addition, the gas sensor may further include a third metal oxidelayer which is formed along the periphery of the metal oxide layer andhas a resistance higher than that of the metal oxide layer.

According to the structure as described above, the position at which thelocal region is to be formed can be controlled at a position in themetal oxide layer except for the third metal oxide layer at which apreferable detection performance of a hydrogen-containing gas can beobtained.

In addition, the local region may be present right under the firstportion of the second conductive layer.

According to the structure as described above, since the local region isformed at a position at which a hydrogen atom dissociated from thehydrogen-containing gas at the first portion is likely to reach, ahydrogen sensor excellent in detection of a hydrogen-containing gas canbe obtained.

In addition, the second conductive layer may be formed of a materialhaving a catalyst function of dissociating the hydrogen atom from thegas molecule.

According to the structure as described above, at a portion of thesecond conductive layer in contact with the local region, a hydrogenatom is dissociated from the hydrogen-containing gas, and the hydrogenatom thus dissociated is bonded to an oxygen atom in the local region ofthe metal oxide layer, so that the resistance between the firstconductive layer and the second conductive layer is decreased.

In addition, the second conductive layer may contain platinum orpalladium.

According to the structure as described above, the second conductivelayer can dissociate a hydrogen atom from the hydrogen molecule by thecatalyst function of platinum or palladium.

In addition, the metal oxide layer may be reversibly changed between ahigh resistive state and a low resistive state in accordance with thevoltage to be applied between the first conductive layer and the secondconductive layer.

According to the structure as described above, the resistive state ofthe metal oxide layer can also be changed besides the change caused by ahydrogen gas. For example, after the metal oxide layer is set in a highresistive state, a gas to be inspected may be brought into contact withthe metal oxide layer. Accordingly, the decrease in resistance can beclearly detected, and hence, the detection performance of ahydrogen-containing gas is improved.

In addition, the gas sensor may further include a measurement circuitmeasuring a current flowing in the metal oxide layer when apredetermined voltage is applied between the first conductive layer andthe second conductive layer. In addition, the gas sensor may furtherinclude an electrical power source which always applies a predeterminedvoltage between the first conductive layer and the second conductivelayer.

According to the structure as described above, as a module componentincluding a measurement circuit and an electrical power source, a highlyconvenient hydrogen sensor can be obtained. In particular, by excellentelectrical power saving of the above hydrogen sensor, a hydrogen gasleakage can be continuously monitored with a small electrical power.

In addition, the first metal oxide and the second metal oxide each maybe either a transition metal oxide or an aluminum oxide.

According to the structure as described above, since a transition metaloxide or an aluminum oxide, each of which is excellent in resistancechange characteristic, is used as the first metal oxide and the secondmetal oxide, a hydrogen sensor excellent in detection of ahydrogen-containing gas can be obtained.

In addition, the first metal oxide and the second metal oxide may beoxides formed of the same transition metal.

According to the structure as described above, since a common materialis used for the first metal oxide and the second metal oxide, a gassensor formable by a simpler manufacturing method can be obtained.

In addition, the first metal oxide and the second metal oxide may beoxides formed of transition metals different from each other.

According to the structure as described above, since the range ofmaterial selection of the first meta oxide and the second metal oxide isincreased, a hydrogen sensor excellent in detection of ahydrogen-containing gas can be obtained.

In addition, the transition metal oxide may be a tantalum oxide, ahafnium oxide, or a zirconium oxide.

According to the structure as described above, since a tantalum oxide, ahafnium oxide, or a zirconium oxide, each of which is excellent inresistance change characteristic, is used as the transition metal oxidedescribed above, a hydrogen sensor excellent in detection of ahydrogen-containing gas can be obtained.

In addition, since the local region is heated by a current flowingbetween the first conductive layer and the second conductive layer, ahydrogen atom is dissociated from the gas molecule at an exposed portionof the second conductive layer, and the hydrogen atom thus dissociatedis bonded to an oxygen atom in the local region of the metal oxidelayer, so that the resistance of the metal oxide layer may be decreased.

According to the structure as described above, the current flowingbetween the first conductive layer and the second conductive layer isconcentrated in the local region having a high degree of oxygendeficiency. As a result, by a small current, the temperature of thelocal region can be increased. Accordingly, by the use of theself-heating and the gas sensitivity in the local region formed in themetal oxide layer, a gas sensor excellent in electrical power saving canbe obtained which can detect a hydrogen-containing gas withoutperforming heating by a heater.

A hydrogen detection method according to one aspect is a hydrogendetection method using a gas sensor which comprises: a first conductivelayer and a second conductive layer, the respective principal surfacesof which face each other; a metal oxide layer disposed so as to be incontact with the principal surface of the first conductive layer and incontact with the principal surface of the second conductive layer; alocal region disposed in the metal oxide layer in contact with thesecond conductive layer and having a high degree of oxygen deficiency ascompared to that of the metal oxide layer: and an insulation layercovering the first conductive layer, the second conductive layer, andthe metal oxide layer. The other principal surface of the secondconductive layer facing the above principal surface thereof is at leastpartially exposed without being covered with the insulation layer; thethickness of a first portion which is the exposed portion of the secondconductive layer is smaller than the thickness of a second portion whichis a portion of the second conductive layer covered with the insulationlayer; and in the hydrogen detection method, when a gas to be inspectedis brought into contact with the first portion of the second conductivelayer, the resistance between the first conductive layer and the secondconductive layer is decreased, so that a gas having a hydrogen atomcontained in the gas to be inspected is detected.

According to the method as described above, by the use of theself-heating and the gas sensitivity in the local region formed in themetal oxide layer, since a hydrogen-containing gas can be detectedwithout performing heating by a heater, hydrogen detection excellent inelectrical power saving can be performed. In addition, since the secondconductive layer includes the first portion having a smaller thicknessthan that of the second portion, a hydrogen atom dissociated from ahydrogen-containing gas brought into contact with the first portionrapidly reaches the metal oxide layer. As a result, the resistancebetween the first conductive layer and the second conductive layer israpidly decreased, and a hydrogen-containing gas can be rapidlydetected.

A fuel-cell vehicle according to one aspect is a fuel-cell vehicle inwhich the gas sensor described above is disposed in at least one of agas tank room in which a hydrogen gas tank is disposed and a fuel-cellroom in which a fuel cell is disposed.

According to the structure as described above, by excellent electricalpower saving of the hydrogen sensor, without remarkably increasing astandby electrical power of the fuel-cell vehicle, a fuel gas leakagecan be always monitored.

For example, when an ignition key is operated, since the presence orabsence of a fuel gas leakage is already judged, compared to the case inwhich after an ignition key is operated, a hydrogen sensor is driven inorder to judge the presence or absence of a fuel gas leak, the start-uptime of a fuel-cell vehicle can be shortened. In addition, for example,even after the fuel-cell vehicle is driven and then stored in a garage,the safety can be improved by continuously monitoring a hydrogen gasleakage.

A hydrogen detection method according to one aspect is a method to judgewhether a hydrogen gas is present or absent in at least one of theoutside of the tank in the gas tank room and the outside of the fuelcell in the fuel-cell room by always applying a predetermined voltage tothe gas sensor in the fuel-cell vehicle.

According to the structure as described above, by excellent electricalpower saving of the gas sensor, without remarkably increasing a standbyelectrical power of the fuel-cell vehicle, a fuel gas leakage can becontinuously monitored.

For example, when an ignition key is operated, since the presence orabsence of a fuel gas leakage is already judged, compared to the case inwhich after an ignition key is operated, a hydrogen sensor is driven inorder to judge the presence or absence of a fuel gas leak, the start-uptime of a fuel-cell vehicle can be shortened. In addition, for example,even after the fuel-cell vehicle is driven and then stored in a garage,since a fuel gas leakage is continuously monitored, the safety can beimproved.

The gas sensor according to the present disclosure may be applied, forexample, to a fuel-cell vehicle, a hydrogen station, and a hydrogenplant.

What is claimed is:
 1. A hydrogen detection method using a gas sensor,wherein the gas sensor includes: a first conductive layer; a secondconductive layer including a first region having a first thickness and asecond region having a second thickness larger than the first thickness;a metal oxide layer disposed between the first conductive layer and thesecond conductive layer, the metal oxide layer including a bulk regionand a local region surrounded by the bulk region, a degree of oxygendeficiency of the local region being higher than that of the bulkregion; and an insulation layer covering the first conductive layer, thesecond region of the second conductive layer, and the metal oxide layerand not covering the first region of the second conductive layer, thehydrogen detection method comprising: allowing a gas to come intocontact with the first region of the second conductive layer; anddetecting a hydrogen gas contained in the gas by detecting a decrease inresistance between the first conductive layer and the second conductivelayer, wherein the first thickness and the second thickness are in adirection perpendicular to a stacking direction between the secondconductive layer and the metal oxide layer.